Track circuit for railroads



July 19 1938. F. x. REES 2,123,964

TRACKCIRCUIT FOR RAILROADS I I Filed May 29, 1955 2 Sheets-Sheet 2 FIG.4.

2 Ohm ballasf 4 Ohm baHasT 10 Ohm ballas'l. I I

' 25 Ohm ballasr 50 Ohm balms? VENTOR BY elm:

Patented July 19, 1938 FATEN'E' OFFEE TRACK CIRCUIT FOR RAILROADS Frank X. Rees, Albany, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Application May 29,

'7 Claims.

This invention relates to improvements in the well-known track circuit for railroads commonly employed in various types of railway signalling and interlocking systems.

The primary object of this invention is to improve the safety and reliability of the track circuit by providing a special organization by which dependable shunting and operation of the track relay are obtained by a higher and more uniform shunting resistance under the varying conditions of rail surface and ballast resistance encountered in practice.

The well-known track circuit is subject to unusual and peculiar operating conditions which materially affect the margins or limitations for its reliable operation. Among other things, there is a relatively low insulating resistance between the track rails, commonly known as ballast leakage or resistance; and this ballast resistance also varies widely with the kind of ballast, drainage, type of ties, and the like. Moreover, this ballast resistance varies greatly for the same track section in wet and dry weather.

Also, the leakage through the ballast or ground around breaks in the track rails and around the insulated joints between the track sections imposes a limitation upon the voltage and resistance of the relay that can be advantageously used, otherwise the track circuit may not perform its important function of detecting broken rails.

Further, the weight of the equipment and condition of the rail surfaces materially modifies the effective resistance of the wheel shunt and impairs the dependable operation of the track re- I lay. In the case of branch lines, station tracks,

switches and cross overs, and the like, the track rails may not be frequently used and may become rusty or dirty to an extent that there is a high contact resistance between the wheels and the rails. Even where tracks are frequently used, the track rails seem to accumulate on the surface something in the nature of a coating or film which is of relatively high resistance at the low voltage commonly used.

With these and other considerations in mind, and by way of explanation of the nature of the invention and without attempting to define its scope, it is proposed to employ a track relay of the usual direct current tractive type but having a substantial value of resistance and/or inductive reactance in series with it, and to supply the track circuit with time spaced unidirectional voltage pulses, in series with the usual limiting resistances; such voltage pulses having peaks materially in excess of the steady operat- 1935, Serial No. 24,074

ing voltage for the track relay and thus tending to reduce the resistance of the wheel contact and establish a wheel shunt of suitable low resistance; the ballast resistance across the track rails, together with an additional shunt or bleeder resistance as may be required, constituting a conducting path into which the relay may discharge current during the time intervals be tween the impressed voltage pulses; and the voltages and resistances being so proportioned that the rate of decay of current through the relay varies with the ballast resistance in such a way as to compensate for the change in the inner-rail impressed voltage with changes in ballast resistance, thereby maintaining a substantially constant average operating current through the track relay throughout a wide range of variation in ballast resistance.

Other characteristic features, attributes, and advantages of the invention will be in part apparent, and in part pointed out as the description progresses.

In the accompanying drawings, Fig. 1 illustrates diagrammatically and conventionally one specific embodiment of the invention; Fig. 2 illustrates a modified arrangement; and Figs. 3 and 4 show curves of current, resistance and the like for explanatory purposes.

Referring to Fig. 1 of the drawings, it is contemplated that the track rails I will be bonded together in the usual way, with insulated joints 2 defining the ends of the track sections, one of which is illustrated. This track section may be a cut-section or block length of a system of automatic block signalling, or automatic train control or the like, or may be a relatively short section at a switch or cross-over in an interlocking plant.

At one end of the track section, preferably at the entering end, a direct current track relay R of the usual type and construction is connected across the track rails i in series with a suitable current limiting device 3, preferably adjustable, which may be an ohmic resistance, or a reactance or choke coil having a relatively high inductance as compared with its ohmic resistance, the choice of the characteristics of this resistance or reactance 3 being dependent upon the nature of the ballast resistance and a number of other factors, as will presently appear.

It is contemplated that the resistance of the track relay R and its connections to the track rails will be some two to six or more times greater than ordinarily used, as for example, a 4 ohm track relay in series with an external resistance or reactance 3 of about 25 ohms. The purpose of such relatively large resistance or reactance of the track relay and its connections is to permit the use of a greater inter-rail potential for reducing the relative resistance of the wheel contact to that part of the track circuit in multiple therewith to provide a wheel shunt of suitable relatively low resistance. Also, such external resistance or reactance 3, tends to make the track relay quicker in releasing its armature when a wheel shunt is applied, since the resistance of the circuit through the relay and the wheel shunt is higher, and the current and flux through the relay may fall at a faster rate, than if a relay of the same resistance and operating current were directly connected across the track rails.

In accordance with this invention, the track circuit is supplied with time-spaced uni-directional voltage pulses, the duration of the pulses and space between them, as well as the peak voltage, being chosen with due regard to the other characteristics of the circuit, so as to obtain the desired operating characteristics of the track circuit. In the embodiment of Fig. 1, a source of alternating current of an ordinary frequency, such as 60 cycles, is employed. Such source of current is shown conventionally as a transformer 4, assumed to have its primary connected to a suitable power supply line, preferably With a motor-generator driven from a battery as a stand-by source. The secondary of this transformer 4 is connected across the track rails i in series with a half-wave rectifier 5, and an adjustable resistance 6 performing the function of the usual limiting resistance used with track circuits. In certain applications of the invention, for reasons hereinafter explained, it is also desirable to employ a bleeder or regulating shunt resistance I, preferably adjustable, which is connected across the track rails as shown, preferably at the feed end.

In the arrangement of Fig. 1, the rectifier 5 serves to impress across the track rails half waves or half cycles of the alternating current voltage, thereby providing the time spaced voltage pulses characteristic of this invention. Certain types of rectifiers, such as the well-known copper-oxide dry plate type, have the property or characteristic that the resistance to the flow of current in the direction in which the rectifier is conducting, varies inversely with the current conducted, such forward resistance of the rectifier increasing as the conducted current decreases. This characteristic or property of the rectifier 5 is a factor in connection with other features to compensate for variations in ballast resistance.

Fig. 2 illustrates a modified arrangement, in which the desired time spaced voltage pulses are obtained from a battery I, or other source of direct current, by the operation of a suitable chopper or interrupter. In the particular arrangement shown, the battery I0 is connected across the track rails through a back contact ll of a relay l2 which is energized from the battery ID by a circuit readily traced on the drawings and including this back contact H, such that this back contact H is intermittently opened and closed to alternately connect and disconnect the battery it) from the track rails. A condenser l3 or equivalent means, is employed in accordance with well-known practice to protect the contact H from injury by sparking. Various other kinds of choppers or interrupters may be employed; and another type of interrupter suitable for this purpose is one comprising a balance wheel or rotary pendulum, such as disclosed for example in the patent to Bossart, No. 1,858,876, May 17, 1932, an interrupter of this type having the advantage that the rate of interruption is not effected by change in the applied voltage.

Considering now the operation of the track circuit organization of this invention, and the theory believed to underlie the results obtained, the impressed voltage applied to the track rails at the feed end comprises uni-directional time spaced pulses or half waves. These voltage pulses supply current to the ballast resistance and to the relay through the limiting resistance 6; and each half wave causes a corresponding increase and decrease of the impressed voltage for the track relay R, such as indicated for explanatory purposes by the curves ER in Fig. 4. As this impressed voltage at the relay end of the track circuit increases, current through the relay increases, with a time lag due to the inductive reactance of the relay, in the manner indicated generally by the curves IR in Fig. 4; and as this impressed voltage falls, the current through the relay decreases. The usual type of direct current track relay intended to be used in this invention comprises the well known core and winding constituting an electro-magnet for attracting a movable armature, usually biased by its weight to assume a retracted position; and such an electro-magnetic structure has considerable self-induction. As the relay current decreases, the accompanying decay of flux induces a voltage in its winding in a direction to sustain the current; and in accordance with established principles, the rate of decrease or decay of the current is dependent upon resistance of the circuit into which the relay may discharge current. It is found that the rate at which the relay current will fall during the half waves where the impressed voltage is cut oil, depends upon the ballast resistance and the shunt or bleeder resistance 1. This is illustrated by the curves in Fig. 4 for different ballast resistances, from which it can be seen that the decay of relay IR during the time intervals between the voltage pulses is at a slower rate with the lower ballast resistances, the greater rate of decay at the higher ballast resistances being also indicated by a reversal of the voltage curves ER.

The pick up and drop-away of the track relay R depends upon what may be termed the average current through it; and it can be seen that the magnitude of this average current depends upon the rate at which the relay current de creases or decays during the half cycles where the impressed voltage is cut off, as well as upon value of the pulsed voltage. In other words, the higher the pulses of impressed voltage, the greater the impressed or applied current; and the slower the rate of decay of relay current, the greater will be the average current through the relay; and vice versa.

In the ordinary track circuit, when the battery voltage and limiting resistance have been selected or adjusted to provide sufiicient current for the track relay under wet ballast conditions with low ballast resistance, if the ballast dries out and its resistance increases, less current flows to the ballast through the limiting resistance, the voltage drop through the limiting resistance then becomes smaller, and a greater voltage is impressed across the track rails to increase the ,-,dry as under wet ballast conditions.

current through the relay. The general trend in this respect is indicated by the curve A in Fig. 3; and it can be seen that, once the ordinary track circuit has been adjusted to hold up the relay under wet ballast conditions, there will be excessive current through the relay under dry ballast conditions, which materially interferes with the proper shunting of the track relay. For these reasons, it is desirable to provide something which will act automatically to maintain a uniform current through the track relay under Varying ballast conditions, so that the energization of the relay is the same at all times, and so that the relay can be shunted safely under both wet and dry ballast conditions. An important feature of this invention is the automatic compensation provided to maintain the relay current substantially uniform for various ballast resistances throughout a wide range at the higher ballast resistances above what may be termed the zone of wheel shunt resistance.

Explaining the theory apparently underlying the operation accomplishing this result, it appears that changes in the voltage applied to the .relay with variation in ballast resistance are accompanied by .a compensating change in the rate of decay of the relay current between the voltage pulses, in such a way that, with suitable proportioning of parts, the average operating current through the relay is maintained substantially uniform throughout a wide range of ballast resistances above the lower values, around 2 ohms per thousand feet of track. The current variations in this respect are indicated generally by the curve B in Fig. 3.

The curves of Fig. 4, based upon oscillograms taken with a typical track circuit organization, may be referred to for explanatory purposes as indicating the voltage and current variations involved in this compensating operation. From the curves of Fig.4, it will be noted that as the ballast resistance increases, the peak voltages ER likewise increase, and are accompanied by higher peaks or humps of relay current IR: but at the same time the change of decay in relay current IR. between the voltage pulses. varies, decreasing more rapidly with the higher ballast resistances, in a manner to: compensate for the higher peak currents, and give a substantially uniform average current, upon. which the operation of the relay depends.

In practicing this invention, the parts; are selected and proportioned to provide adequate operating current for the relay under the most unfavorable or wet ballast conditions to be encountered; but as. the ballast dries out, less current is supplied to the track circuit as a whole, and the voltage drop through the limiting resistance decreases to raise the voltage impressed upon the relay, such increase in ballast resist ance is accomplished by a more rapid decay of relay current between the pulses. of impressed voltage, so that the average current remains substantially constant, rather than increase steadily with increased ballast resistance as in the ordinary track circuit. Consequently, the track relay'R is not over energized under dry ballast conditions and may be shunted as surely under The lines PU and DA in Fig. 3 indicate the pick-up and drop-away currents for the organization illustrated by these curves; and it will be noted from the curve B for this invention that the current .through relay above the shunting zone remains nearly uniform slightly above the pick-up or normal operating current.

In some applications of the invention, it is found that there is what may be termed over compensation, such that the current through the relay decreases at the higher ballast resistances to the point where it will fail to hold up its armature. In other words, the curve B of Fig. 3 may in some instances droop at the higher values of ballast resistance below the drop away line DA. This appears to be due to a relatively greater change in relay current between the voltage pulses for higher ballast resistances, than changes in impressed voltage, with the result that the average current decreases at the higher values of ballast resistances. In such cases, the shunt or bleeder resistance 1 across the track rails is employed to provide what may be considered tobe an artificial ballast resistance or leakage for the purpose of maintaining a sufficiently low resistance of the discharge circuit for the track relay under very dry ballast conditions to avoid too quick a decay in the relay current between the voltage pulses and thereby maintain the average current. This shunt or bleeder resistance 1 is preferably adjustable; and by proper selection or adjustment of this resistance'l, together with the adjustment of the usual limiting resistance 6, it appears that the desired regulation of relay current may be obtained for track circuits for Various lengths and ballast resistance throughout the full operating range encountered in practice.

The maintenance of substantially constant current through the relay for different resistances seems to be helped also by the currentresistance characteristics of the copper-oxide rectifier 5 in the arrangement of Fig. 1, the effect of the rectifier in this respect, however, being apparently dependent upon a number of related factors and dilficult to explain and evaluate. A source of alternating current, with a half-wave rectifier 5, is a convenient way of obtaining the time spaced impulses of impressed voltage; and with such an arrangement, the operating characteristics of the rectifier contribute to or modify the current regulating characteristics of the track circuit.

For example, assumirng a setting for wet ballast conditions, when the ballast dries out and less current is supplied to the track, the forward resistance of the rectifier, i. e. the resistance in the direction in which it is conducting, increases and in eifect adds to the limiting resistance 6 to maintain a voltage drop with the smaller current to give nearly the same impressed voltage across the track rails. For this reason, and in this way, the characteristics of the rectifier may be helpful in obtaining the desired current regulation.

Another important feature of this invention is that the peak voltages of the half cycles of alternating current or other time spaced voltage pulses are much in excess of the steady direct current voltage that would be required to produce the same general average operating current through the relay. In. other words, the peak voltages applied across the track rails in accordance with this. invention are much greater than the steady voltage that would be used with the same resistance and type of track relay. Such peak voltages very materially assist in reducing or breaking down the resistance of the wheel contacts and establish an effective low resistance wheel shunt, more particularly with dirty or.

dusty track rails, or with light-weight equipment, such as gas-electric cars.

The resistance of the wheel contact is found to vary greatly with the conditions of the rail surface and the weight of the equipment; and it appears that voltages much higher than the relatively small voltages of about two volts normally employed for track circuits are necessary to reduce the resistance of the wheel contact in many cases to a point where actual shunting of the track relay is obtained. The peak voltages provided by this invention, which are many times those commonly used in track circuits, apparently break down the resistance at the wheel contact, perhaps due to an ionization eifect, and enable effective shunting with dirty or rusty track rails and light equipment in a manner not obtainable with the ordinary track circuit arrangement.

In this connection, it will be noted that the peak voltages are periodically applied at frequent intervals, and are available to break down the resistance of the wheel contact, as the car or train moves along the track and its wheels contact with the rails at different points.

Further, the use of a pulsating current, obtained from an alternating current half-wave rectifier or from a battery by an interrupter, as distinctive from a steady current for energizing the track relay, acts to improve the pick-up and drop-away characteristics of the relay. In other words, a direct current tractive type relay of the usual construction will pick up on less current and drop-away on a greater current when energized with half cycles of rectified alternating current or equivalent time spaced voltage pulses, than if energized with a steady uni-directional voltage from .a battery or the like. This is attributed to the fluctuating nature of the energizing current which tends to initiate armature movement at critical average current values in a manner that does not occur with steady energization. Also, the movement of the armature of the relay is accompanied by a flux change which appears to be a contributing factor to the superior ratio of drop-away current to pick-up current obtained by the use of pulsating energization of the track relay in accordance with this invention. 1

From the foregoing it will be appreciated that this invention involves certain selection and proportioning of parts in the complete organization; and the various voltages, resistances, and inductive reactances are so complexly inter-related that a variation or change in the value of one factor tends to modify one or more of the other factors. The quantitative values or relationship indicated by the explanatory curves of Figs. 3 and 4 are merely typical; and it should be understood that the invention is not limited to any such particular quantitative relations.

Generally speaking, the underlying principles and advantages of the invention may be obtained when the limiting resistance 6 and the shunt or bleeder 1', if used, are selected to conform with the length of the track circuit, condition of the ballast, range of variation in ballast resistance, and the like; the external resistance or inductive reactance 3 in series with the track relay R is selected or adjusted to require peak voltages suitable for reducing the resistance of the wheel contact, and also utilize as high a wheel shunt resistance as desired; and these various factors are relatively proportioned to provide the necessary average current through the track relay under wet ballast conditions, and to maintain such average current substantially constant throughout the range of variation in ballast resistance to be encountered.

The nature of the invention is such that various modifications, adaptations, and additions may be made in the particular embodiments shown and described without departing from the invention.

What I claim is:-

1. A track circuit for railroads comprising, in combination with a direct current track relay of the usual tractive armature type connected across the rails at one end of a track section, a source of alternating current connected across the track rails in series with a half-wave rectifier and a limiting resistance at the other end of said track section, and an ohmic shunt bleeder resistance connected across the track rails at said other end of said track section, said resistances being proportioned with regard to the operating current and inductive reactance of the relay to cooperate with the ballast resistance and maintain approximately constant current through the relay for a wide range of variation in ballast resistance.

2. In a track circuit for railroads, the combination with a direct current track relay of the usual tractive armature type connected across the track rails at one end of the track section, a limiting resistance, means for impressing across the track. rails at the other end of said section through said limiting resistance time spaced unidirectional voltage pulses having peak voltages greater than the steady operating voltage for the track relay, a shunt bleeder resistance connected across the track rails at said other end of said track section, said shunt resistance and ballast resistance forming a conducting path into which the track relay may inductively discharge current during the time periods of cessation of impressed voltage pulses, the resistance of said conducting path being independent of the intensity of the current flowing so that the rate of decay of the relay current during the intervals between said voltage pulses varies with the ballast resistance to compensate for change in impressed voltage with change in ballast resistance.

3. A track circuit for railroads comprising, a direct current track relay of the tractive armature type connected across the track rails at one end of the track section in series with an external ohmic resistance, a limiting resistance, means for impressing time spaced pulses of unidirectional voltage across the track rails at the other end of said track section in series with said limiting resistance, and an ohmic bleeder resistance connected across the track rails, said bleeder resistance co-acting with the ballast resistance to provide a conducting path into which the relay may inductively discharge current when the impressed voltage pulse is cut off so as to partially compensate for the change in impressed voltage with the change in ballast resistance, the resistance of said conducting path varying with the ballast resistance but being independent of the intensity of the current flowing.

4. A track circuit for railroads comprising,- a direct current track relay of the usual tractive armature type connected across the track rails at one end of the track section in series with an external ohmic resistance, a source of alternating current, an adjustable limiting resistance, a half-wave rectifier, and means connecting said source of alternating current across the track rails at the other end of said track section in series with said limiting resistance and said halfwave rectifier, an ohmic bleeder resistance connected across the track rails at said other end of the track section, the self-induction of said track relay tending to maintain current in its winding to keep its armature attracted between the half cycles of impressed voltage by discharging its current of self-induction into the ballast resistance in multiple with said bleeder resistance, and the peak voltages of the half cycles of impressed voltage suitable for providing an average current through said track relay comparable with its steady operating current being materially greater than the voltage for such steady operating current, whereby peak voltages across the track rails suitable for breaking down any contact resistance of the wheel shunt are available without a correspondingly greater average energization of the track relay which would interfere with its shunting.

5. A track circuit for railroads comprising, a direct current track relay of the usual tractive armature type having its winding connected across the track rails at one end of a track section, a limiting resistance, and means for supplying time spaced uni-directional voltage pulses through said limiting resistance across the track rails at the other end of said section, an ohmic bleeder resistance many times greater than the ballast resistance connected across the track rails, the self-induction of said relay maintaining current through its winding between such voltage pulses by discharging into the ballast resistance in multiple with said bleeder resistance, the rate of decay of said current of self-induction between the voltage pulses varying with changes in ballast resistance to compensate for the variations in current supplied to said track relay by the voltage pulses for difierent ballast resistances, whereby approximately the same average current through the track relay is maintained for a wide variation in ballast resistance.

6. A track circuit for railroads comprising, a source of time spaced uni-directional voltage pulses, an adjustable limiting resistance, an ohmic shunt bleeder resistance many times greater than the ballast resistance, means for connecting said source of voltage across the track rails at one end of the track section in series with said limiting resistance and in multiple with said bleeder resistance, and a direct current track relay of the usual tractive armature type connected across the track rails at the other end of said track section in series with an external ohmic resistance, whereby peak voltages materially greater than the steady operating voltage for the track relay are applied across the track rails to supply sufficient energizing current for the track relay and are available for breaking down the contact resistance of a wheel shunt and also approximately the same average current in the track relay is maintained for a wide variation in ballast resistance.

'7. A track circuit for railroads comprising, a direct current track relay of the usual tractive armature type having its winding connected across the track rails at one end of the track section, an external ohmic resistance in series with said track relay, an adjustable limiting resistance, means including a source of alternating current and a half-wave rectifier for applying half waves of impressed voltage across the track rails at the other end of said track section in series with said limiting resistance, and an ohmic bleeder resistance many times the ballast resistance connected across the track rails at the other end of said track section, the self-induction of said track relay tending to maintain current in its winding between the half waves of impressed voltage but discharging current into a conducting path including the ballast resistance in multiple with said bleeder resistance, the resistance of said conducting path being independent of the intensity of the current flowing so that the rate of decay of such current of self-induction in the relay winding varies with variations in the ballast resistance.

FRANK X. REES. 

